(a) Field of the Invention
The present invention relates to a crystallization mask, a crystallization method, and a method of manufacturing a thin film transistor including crystallized semiconductor.
(b) Description of Related Art
A thin film transistor array panel is used for a display device such as a liquid crystal display (LCD) or an organic light emitting display (OLED), which includes a plurality of pixels independently driven by thin film transistors (TFTs).
In general, an LCD includes two panels having field generating electrodes and a liquid crystal layer interposed therebetween. This LCD displays desired images by applying electric field using the electrodes to the liquid crystal layer with dielectric anisotropy and adjusting the strength of the electric field to control the amount of light passing through the panels. In this case, TFTs are used for controlling signals transmitted to the electrodes.
An OLED is a self emissive display device, which displays images by exciting an emissive organic material to emit light. The OLED includes an anode (hole injection electrode), a cathode (electron injection electrode), and an organic light emission layer interposed therebetween. When the holes and the electrons are injected into the light emission layer, they are recombined and pair annihilated with emitting light. The light emission layer further includes an electron transport layer (ETL) and a hole transport layer (HTL) as well as an electron injecting layer (EIL) and a hole injecting layer (HIL) for enhancing the light emission. Each pixel of the OLED includes two TFTs, i.e., a switching TFT and a driving TFT. The current for light emission is driven by the driving TFT and the mount of the current driven the driving TFT is controlled by the data signals from the switching TFT.
The most commonly used TFTs for an LCD adapts amorphous silicon as a semiconductor layer.
An amorphous silicon TFT has mobility of about 0.5 to 1 cm2/Vsec, which is suitable for a switching element of an LCD. However, it is not sufficient for a driving circuit of a display device.
In order to overcome such a problem, an organic EL or a polysilicon TFT LCD using a polysilicon with electron mobility of 20 to 150 cm2/Vsec as a semiconductor layer has been developed. The relatively high electron mobility polysilicon TFT enables to implement a chip in glass technique that a display panel embeds its driving circuits.
In recent years, one of the most widely used methods of forming a polysilicon thin film on a glass substrate with a low melting point is an eximner laser annealing technique. The technique irradiates light with the wavelength, which can be absorbed by amorphous silicon, from an eximer laser into an amorphous silicon layer deposited on a substrate to melt the amorphous silicon layer at 1,400° C., thereby crystallizing the amorphous silicon into polysilicon. The crystal grain has a relatively uniform size ranging about 3,000–5,000, and the crystallization rime is only about 30–200 nanoseconds, which does not damage the glass substrate. However, there are disadvantages that non-uniform grain boundaries decrease the uniformity for electrical characteristics between the TFTs and make it hard to adjust the microstructure of the grams.
To solve these problems, a sequential lateral solidification process capable of adjusting the distribution of the grain boundaries has been developed. The process is based on the fact that the grains of polysilicon at the boundary between a liquid phase region exposed to laser beam and a solid phase region not exposed to laser beam grow in a direction perpendicular to the boundary surface. A mask having a slit pattern is provided, and a laser beam passes through transmittance areas of the mask to completely melt amorphous silicon, thereby producing liquid phase regions arranged in a slit pattern. Thereafter, the melted amorphous silicon cools down to be crystallized, and the crystal growth starts from the boundaries of the solid phase regions not exposed to the laser beam, and proceeds in the directions perpendicular to the boundary surface. The grains stop growing when they encounter each other at the center of the liquid phase region. This process is repeated after moving the slit pattern of the mask in the direction of the grain growth, and thus the sequential lateral solidification covers the whole area. The sizes of the grains can be as much as the widths of the slit pattern.
After the sequential lateral solidification, protrusions of protuberances of about 400–1,000 Å are formed on the surface of the polysilicon layer along the grain boundaries. These causes stress on the boundary surface of a gate insulating layer to be formed on the semiconductor layer. The stress in this process is found to be ten times more than that in the eximer laser annealing, and this results in degrading the characteristics of the TFTs.